Catalyst component for the (CO)polymerization of olefins

ABSTRACT

Solid catalyst components comprising Ti,Mg, halogen and an internal electron-donor compound selected from the 1,3-diethers of formula (I) in which R is a C 1 -C 10  alkyl group, R 1  is a linear or branched primary alkyl radical having at least three carbon atoms, optionally containing a heteroatom, and R 2  is a secondary alkyl or cycloalkyl radicals different from i-propyl, optionally containing a heteroatom. The catalysts obtained by using as internal electron-donor compound the said 1,3-diethers display in the (co)polymerization of olefins an excellent balance of activity and stereospecificity that cannot be reached with the ethers known in the art.

The present invention relates to solid components of catalysts for the(co)polymerization of olefins and the catalysts obtained therefrom. Inparticular, the present invention relates to Ziegler-Natta catalystcomponent suitable for the (co)polymerization of propylene andcomprising Ti, Mg, halogen and a specific electron-donor compound. Thecatalysts including titanium compounds supported on magnesium halidesare well known in the art. Catalysts of this type are described in theU.S. Pat. No. 4,298,718. Said catalysts comprise titanium tetrahalidessupported on halides of magnesium. Although the catalysts have highactivity in the polymerization of alpha olefins like propylene, they arenot very stereospecific. Improvements to stereospecificity have beenmade by adding electron-donor compounds to the solid catalyst component.Substantial improvements were made using, in addition to theelectron-donor present in the solid component, an electron-donor addedto the aluminum alkyl co-catalyst component (U.S. Pat. No. 4,107,414).The catalysts modified in this manner although being highlystereospecific (Isotactic Index about 94-950%) still do not showsufficiently high levels of activity. Significant improvements inactivity and stereospecificity were obtained by preparing the solidcatalytic component according to the technique described in U.S. Pat.No. 4,226,741. High level performance in catalyst activity as well asstereospecificity have been obtained with the catalysts described in theEuropean patent No.045977. Said catalysts have as solid catalystscomponent, a magnesium halide on which is supported a titanium halide,preferably TiCl₄, and an electron-donor compound, selected from specificclasses of carboxylic acid esters, and, as co-catalyst component, asystem formed of an Al-trialkyl compound and a silicon compoundcontaining at least one Si—OR bond (R hydrocarbyl radical). Neverthelessthe results described above, research activities have been continuedwith the purpose of modifying and/or improving the performance of thementioned catalysts. The European patent No.0361494 describes solidcatalyst components for the polymerization of olefins comprising, as aninternal electron-donor compound, an ether containing two or more ethergroups and having specific reaction characteristics toward the anhydrousmagnesium chloride and TiCl₄. The catalysts obtained from the reactionof said catalyst components with an Al-alkyl compound exhibit a so highactivity and stereospecificity in the polymerization of olefins, thatthe use of an external electron-donor can be avoided. Due to the factthat the field of the polymerization of olefins is highly competitive itis clear that it is always felt the need of improving the performanceswith the respect to what is already known in the art Now the applicanthas surprisingly found new catalyst components having such improvedcharacteristics. In fact, it has been found that the catalysts obtainedby using as internal electron-donor compounds the 1,3-diethers offormula (1)in which R is a C₁-C₁₀ alkyl group, R₁ is a linear or branched primaryalkyl group containing at least three carbon atoms, and optionallycontaining a heteroatom and R₂ is a secondary the alkyl or cycloalkyldifferent from i-propyl and optionally containing a heteroatom, display,in the (co)polymerization of olefins, an excellent balance of activityand stereospecificity that cannot be reached with the ethers known inthe art.

The present invention provides a solid catalyst component for the(co)polymerization of olefins CH₂═CHR, in which R is hydrogen orhydrocarbyl radical with 1-12 carbon atoms comprising Mg, Ti, halogenand the 1,3-diethers of formula (I). Particularly preferred are thesolid catalyst components comprising a titanium compound, containing atleast one Ti-halogen bond, and an internal electron-donor compoundchosen from the above mentioned 1,3-diethers, supported on magnesiumhalide. In another preferred embodiment of the said solid catalystcomponents Mg-halide is in active form The active form of the magnesiumhalides present in the catalyst components of the invention isrecognizable by the fact that in the X-ray spectrum of the catalystcomponent the major intensity reflection which appears in the spectrumof the nonactivated magnesium halides (having surface area smaller than3 m²/g) is no longer present, but in its place there is a halo with theposition of the maximum intensity shifted with respect to the positionof the major intensity reflection, or by the fact that the majorintensity reflection presents a half-peak breadth at least 30% greaterthat the one of the corresponding reflection of the nonactivated Mghalide. The most active forms are those in which the halo appears in theX-ray spectrum of the solid catalyst component. Among the magnesiumhalides, the chloride is the preferred compound. In the case of the mostactive forms of the magnesium chloride, the halo appears in place of thereflection which in the spectrum of the nonactivated magnesium chlorideis situated at the interplanar distance of 2.56 Å. In the 1,3-diethersof formula (I) R is preferably a methyl group, R₁ is preferably a C₄-C₇linear or branched primary alkyl radical, optionally containing aheteroatom, and R₂ is preferably a cycloalkyl or a (R₃)₂—CH— radical,where the R₃ groups, same or different, are C₁-C₁₀ linear alkylradicals, provided that they are not contemporarily CH₃; said R₃ groupsoptionally containing a heteroatom selected from halogens, in particularF. Particularly preferred are the R₂ groups selected from C₃-C₅secondary alkyl radicals or Cs-C₇ cycloalkyl. R₁ is preferably selectedfrom the group consisting of n-butyl, n-pentyl, n-hexyl n-heptyl,i-butyl and i-pentyl R₂ is preferably selected from the group consistingof sec-butyl, 3-pentyl (1-trifluoromethyl)ethyl cyclopentyl, cyclohexyland cycloheptyl.

Examples of representative 1,3 diethers that are included in the aboveformula (I) are: 2-n-propyl-2-sec-butyl-1,3-dimethoxypropane,2-n-butyl-2-sec-butyl-1,3-dimethoxypropane,2-n-butyl-2-(3-pentyl)-1,3-dimethoxypropane,2-n-pentyl-2-sec-butyl-1,3-dimethoxypropane,2-n-propyl-2cyclopentyl-11,3-dimethoxypropane,2-n-propyl-2-cyclohexyl-1,3-dimethoxypropane,2-n-butyl-2-cyclopentyl-1,3-dimethoxypropane,2-n-butyl-2-cyclohexyl-1,3-dimethoxypropane,2-n-butyl-2-cycloheptyl-1,3-dimethoxypropane,2-n-pentyl-2-cyclopentyl-3-dimethoxypropane,2-n-pentyl-2-cyclohexyl-1,3-dimethoxypropane,2-n-hexyl-2-cyclopentyl-1,3-dimethoxypropane,2-n-heptyl-2-cyclopentyl-1,3-dimethoxypropane,2-1-butyl-2-(3-pentyl)-1,3-dimethoxypropane,2-n-butyl-2-cycloheptyl-1,3-dimethoxypropane,2-1-pentyl-2-cyclopentyl-1,3-dimethoxypropane,2-1-butyl-2-(1-trifluoromethyl)ethyl-1,3-dimethoxypropane. 1,3-diethersparticularly preferred are: 2-n-butyl-2-sec-butyl-1,3-dimethoxypropane,2-n-butyl-2-(3-pentyl)-1,3-dimethoxypropane,2-n-pentyl-2-sec-butyl-1,3-dimethoxypropane,2-n-propyl-2-cyclohexyl)-1,3-dimethoxypropane,2-n-butyl-2-cyclohexyl-1,3-dimethoxypropane,2-n-pentyl-2-cyclohexyl-1,3-dimethoxypropane,2-n-hexyl-2-cyclopentyl-1,3-dimethoxypropane,2-1-butyl-2-cycloheptyl-1,3-dimethoxypropane,2-1-pentyl-2-cyclopentyl-1,3-dimethoxypropane,2-i-butyl-2-(1-trifluoromethyl)ethyl-1,3-dimethoxypropane.

The 1,3-diethers of the present invention can be prepared according tothe methods disclosed in the European patent application No.0361493.Said diethers, used in the preparation of Ziegler-Natta catalysts, aregenerally syntesized by the reaction of alkylant agents with the diolscorresponding to the above diethers. A way of synthesis of said diolsconsists in the reduction of the corresponding malonates.

The preparation of the solid catalyst components can be carried outusing various methods.

For example, the magnesium halide (preferably used in a form containingless than 1% of water), the titanium compound and the electron-donorcompound are milled together under conditions that cause the activationof the magnesium halide; the milled product is then caused to react oneor more times with TiCl₄ in excess, optionally in the presence of anelectron-donor, at a temperature ranging from 80 to 135° C., and thenrepeatedly washed with a hydrocarbon (such as hexane) until no chlorineions can be detected in the wash liquid. According to another method,the anhydrous magnesium halide is preactivated according to knownmethods and then reacted with an excess of TiCl₄ containing theelectron-donor compound and optionally an aliphatic, cycloaliphatic,aromatic or chlorinated hydrocarbon solvent (for example hexane,heptane, cyclohexane, toluene, ethylbenzene, chlorobenzene,dichloroethane). In this case also the operation takes place at atemperature between 80° and 135° C. The reaction with TiCl₄, in thepresence or absence of an electron-donor, is optionally repeated and thesolid is then washed with hexane to eliminate the nonreacted TiCl₄.According to a preferred method, a MgCl₂.nROH adduct (particularly inthe form of spheroidal particles) where n is generally from 1 to 6, andROH is an alcohol, preferably ethanol, is caused to react with an excessof TiCl₄ containing the electron-donor compound and optionally one ofthe above mentioned hydrocarbon solvents. The reaction temperatureinitially is from 0° to 25° C., and is then increased to 80-135° C.Then, the solid is reacted once more with TiCl₄, in the presence orabsence of the electron-donor, separated and washed with a hydrocarbonuntil no chlorine ions can be detected in the wash liquid. According toyet another method, magnesium alcoholates and chloroalcoholates (thechloroalcoholates can be prepared particularly as described in U.S. Pat.No. 4,220,554) are caused to react with TiCl₄ in excess containing theelectron-donor compound, operating under the reaction conditions alreadydescribed. According to a further method, complexes of magnesium halideswith titanium alcoholates (the MgCl₂.2Ti(OC₄H₉)₄ complex is a typicalexample) are caused to react, in a hydrocarbon solution, with TiCl₄ inexcess containing the electron-donor compound; the separated solidproduct is reacted again with an excess of TiCl₄, in the presence orabsence of electron-donor, and then separated and washed with hexane.The reaction with TiCl₄ is carried out at a temperature ranging from 80°to 130° C. According to a variance of the latter method, the MgCl₂ andtitanium alcoholate complex is caused to react in a hydrocarbon solutionwith polyhydrosiloxane; the separated solid product is reacted at 50° C.with silicon tetrachloride containing the electron-donor compound; thesolid is then reacted with TiCl₄ in excess, in the presence or absenceof electron-donor, operating at 800-130° C.

Independently from the specific preparation method, after the lastreaction with TiCl₄ in the presence of the electron-donor, it ispreferable to separate the solid obtained (by way of filtration, forexample), and cause it to react with an excess of TiCl₄ at temperaturesranging from 800 to 135° C., before washing it with the hydrocarbonsolvent.

Finally, it is possible to cause to react TiCl₄ in excess and containingthe electron-donor with porous resins such as partially cross-linkedstyrene-divinylbenzene in spherical particle form, or porous inorganicoxides such as silica and alumina, impregnated with solutions ofmagnesium compounds or complexes soluble in organic solvents. The porousresins which can be used are described in the European patentapplication No.0344755. The reaction with TiCl₄ is carried out at80-100° C. After separating the excess of TiCl₄, the reaction isrepeated and the solid is then washed with a hydrocarbon. TheMgCl₂/electron-donor molar ratio used in the reactions indicated abovegenerally ranges from 2:1 to 30:1, preferably from 4:1 to 12:1. Theelectron-donor compound is fixed on the magnesium halide in a quantitygenerally ranging from 1 to 25% molar with respect to MgCl₂. Inparticular, the 1,3-diethers of formula (I) are present on the catalystcomponent in a quantity generally ranging from S to 30% weight,preferably from 8 to 25% weight. In the solid catalyst components theMg/Ti molar ratio is generally from 30:1 to 3:1; in the componentssupported on resins or on inorganic oxides the ratio can be differentand usually ranges from 20:1 to 2:1. The titanium compounds that can beused for the preparation of the catalyst components are the halides orthe compounds of formula TiX_(n)(OR⁴)_(4-n), where 0<n≦3, X is halogen,preferably chlorine, and R is C₁-C₁₀ hydrocarbon group. The titaniumtetrachloride is the preferred compound. Satisfactory results can alsobe obtained with the trihalides, particularly TiCl₃ HR TiCl₃ ARA, andwith the halogen alcoholates such as TiCl₃ OR, where R is a C₁-C₁₀hydrocarbon radical. The solid catalyst components of the inventionform, by reaction with the Al-alkyl compounds, catalysts which can beused in the (co)polymerization of CH₂═CHR olefins, where R has themeaning given above. Therefore, the present invention provides acatalyst for the (co)polymerization of olefins comprising the reactionproduct of:

-   -   a) a catalyst component as defined above;    -   b) an Al-alkyl compound and optionally    -   c) an electron-donor compound.

The Al-alkyl compounds comprise Al-trialkyls such as Al-triethyl,Al-triisobutyl, Al-tri-n-butyl, Al-trioctyl and alkyl aluminum halides,such as AlEt₂Cl and Al₂Et₃Cl₃. Also linear or cyclic Al-alkyl compoundscontaining one or more Al atoms bonded to one another with O, N, or Satoms can be used.

The Al-Alkyl compound is used in Al/Ti ratios generally ranging from 1to 1000.

The trialkyl compounds can also be used in blends with Al-alkyl halidessuch as AlEt₂Cl and AlEt_(3/2)Cl_(3/2). The polymerization of theolefins is carried out according to known methods operating in liquidphase constituted by one or more monomers, or by a solution of one ormore monomers in an aliphatic or aromatic hydrocarbon solvent, or in gasphase, or also by combining polymerization stages in liquid phase and ingas phase. The (co)polymerization temperature is usually from 0° to 150°C.; particularly from 60° to 100° C. The operation occurs at atmosphericpressure or higher. The catalysts can be precontacted with smallquantities of olefins (prepolymerization). The prepolymerizationimproves the performance of the catalysts as well as the morphology ofthe polymers. The prepolymerization is carried out maintaining thecatalysts in suspension in a hydrocarbon solvent (hexane or heptane, forexample), adding an olefin, and operating at temperatures ranging fromroom temperature to 60° C. producing a quantity of polymer generallyfrom 0.5 to 50 times the weight of the catalyst. It can also be carriedout in liquid monomer, under the temperature conditions indicated above,and producing quantities of polymer which can reach 1000 g per g ofcatalytic component As explained above the said catalysts show anexcellent balance of activity and stereospecificity in the stereoregularpolymerization of olefins even without the use of an external donor. Ifdesired however, this latter can be added to the Al-alkyl compound and,in this case, the said external electron-donor can preferably beselected from the group consisting of silicon compounds containing atleast one Si—OR bond (R is a hydrocarbon radical);2,2,6,6-etramethylpiperidine; 2,6-diisopropylpiperidine; carboxylic acidesters, such as ethylparatoluate and ethylbenzoate, and di- andpolyethers. Preferably, the silicon compounds have the formula R⁵_(q)Si(OR⁶)_(4-q) where q is from 1 to 3, the R⁵ radical or radicals,same or different, are C₁-C₁₂ alkyl C₃-C₁₂ cycloalkyl, C₆-C₁₂ aryl,C₇-C₁₂ alkaryl or aralkyl radicals, R⁷—N—R⁸ radicals, where R⁷ and R⁸are the same or different and have the same meaning defined above forR⁵, or are bonded to each other to form a cyclic structure; the R₁radicals are the same or different and are C₁-C₆ alkyl radicals.Optionally the R to R₁ radicals can contain one or more halogens, inparticular Cl and F, as substitutes for hydrogen atoms.

Examples of said compounds are: (tert-butyl)₂Si(OCH₃)₂;(cyclohexyl)₂Si(OCH₃)₂; (isopropyl)₂Si(OCH₃)₂; (sec-butyl)₂Si(OCH₃)₂;(cyclohexyl)(methyl)Si(OCH₃)₂; (cyclopentyl)₂Si(OCH₃)₂;(isopropyl)(methyl)Si(OCH₃)₂; (n-butyl)₂Si(OCH₃)₂; (isobutyl)₂Si(OCH₃)₂;(sec-butyl)₂Si(OCH₃)₂; (tert-butyl)(methyl)Si(OCH₃)₂;(tert-amyl)(methyl)Si(OCH₃)₂; (tert-hexyl)(methyl)Si(OCH₃)₂;(2-norbornyl)(methyl)Si(OCH₃)₂; (tert-butyl)(cyclopentyl)Si(OCH₃)₂;(2-norbornyl)(cyclopentyl)Si(OCH₃)₂; (tert-butyl)Si(OCH₃)₃;(tert-butyl)Si(OC₂H₅)₃; (2-norbornyl)Si(OCH₃)₃; (2-norbornyl)Si(OC₂H₅)₃;(tert-hexyl)Si(OCH₃)₃; (tert-hexyl)Si(OC₂H₅)₃;(tert-butyl)(2-methylpiperidyl)Si(OCH₃)₂;(tert-butyl)(3-methylpiperidyl)Si(OCH₃)₂;(tert-butyl)(4-methylpiperidyl)Si(OCH₃)₂;(tert-hexyl)(piperidyl)Si(OCH₃)₂; (tert-hexyl)(pyrrolidinyl)Si(OCH₃)₂;(methyl)(3,3,3-trifluoropropyl)Si(OCH₃)₂;(isopropyl)(3,3,3-trifluoropropyl)Si(OCH₃)₂;(n-butyl)(3,3,3-tuoropropyl)Si(OCH₃)₂;(isobutyl)(3,3,3-trifiuoropropyl)Si(OCH₃)₂;(sec-butyl)(3,3,3-trifluoropropyl)Si(OCH₃)₂;(tert-butyl)(3,3,3-trifluoropropyl)Si(OCH₃)₂;(3,3,3-trifluoropropyl)(piperidyl)Si(OCH₃);(3,3,3-trifluoropropyl)(2-methylpiperidyl)Si(OCH₃₂;(3,3,3-trifluoropropyl)(2-methylpiperidyl)Si(OCH₃)₂;(3,3,3-trifluoropropyl)(3-methylpiperidyl)Si(OCH₃)₂;(3,3,3-trifluoropropyl)(4-methylpiperidyl)Si(OCH₃)₂;(3,3,3-trifluoropropyl)₂Si(OCH₃)₂.

The molar ratio of the Al-alkyl compound to the external electron-donorgenerally is from 2:1 to 100:1 and preferably from 10:1 to 30:1; saidratio can be broader, for example from 0.5:1 to 100:1, during theprepolymerization phase. The catalysts find particular application inthe (co)polymerization of CH₂=CHR olefins where R is hydrogen or a 1-6carbon alkyl or aryl radical. In particular, said catalysts are suitableused for the stereospecific polymerization of propylene or itscopolymerization with ethylene or other α-olefins such as 1-butene,1-hexene and 1-octene. The following examples are given in order toillustrate and not limit the invention. Unless otherwise indicated, thepercentages in the examples are expressed by weight.

The melt flow rate (MFR) for polypropylene is determined according toASTM D1238, condition L. The intrinsic viscosity [η] is determined intetrahydronaphthalene at 135° C. (ASTM 2857-70). In order to determinethe fraction insoluble in xylene at 25° C. (X.I.%), 2.5 g of polymer aredissolved under agitation in 250 ml of xylene at 135° C., and after 20minutes it is allowed to cool to 25° C. After 30 minutes theprecipitated polymer is filtered and dried at reduced pressure at 80° C.until constant weight is reached.

Synthesis of 1,3-diethers (I)

The 1,3-diethers of formula (1) used in the present invention have beenprepared by alkylation of diethyl malonates (see J. March in “AdvancedOrganic Chemistry” V Ed.; 1992; pp. 464-468) followed by reduction tothe corresponding diols (see “ibid” pp 1214) and methylation (see R. C.Larock in “Comprehensive organic transformations” Ed VCH 1989 pp.445-472).

EXAMPLE 1

Into a 500 ml four-necked round flask, purged with nitrogen, 250 ml ofTiCl₄ were introduced at 0° C. While stirrng, 10.0 g of microspheroidalMgCl₂.2.8C₂H₅₀H (prepared according to the method described in ex.2 ofU.S. Pat. No. 4,399,054 but operating at 3,000 rpm instead of 10,000)and 7.5 mmoles of 2-n-butyl-2-sec-butyl-1,3-dimethoxypropane were added.The temperature was rised to 100° C. and maintained for 120 min. Then,the sting was discontinued, the solid product was allowed to settle andthe supernatant liquid was siphoned off. 250 ml of fresh TiCl₄ wereadded. The mixture was reacted at 120° C. for 60 min and, then, thesupernatant liquid was siphoned off. The solid was washed six times withanhydrous hexane (6×100 ml) at 60° C. Finally, the solid was dried undervacuum and analyzed. The catalyst component obtained in this mannercontains: Ti=4.6%; Mg=15.9%;2-n-butyl-2-sec-butyl-1,3-dimethoxypropane=22.1%. In a 4 literautoclave, previously purged with gaseous propylene at 70° C. for 1hour, are introduced at room temperature and in propylene flow 70 ml ofanhydrous n-hexane containing 7 mmoles of aluminum triethyl and 4 mg ofthe solid catalyst component prepared as described above. The autoclaveis closed, 1.7 Ni of hydrogen and 1.2 kg of liquid propylene areintroduced; the agitator is put in motion and the temperature isincreased to 70° C. in a period of 5 minutes. After 2 hours at 70° C.,the agitation is interrupted, the nonpolymerized monomer is removed, andthe content is cooled to room temperature.

The polymer yield is 103 Kg of polypropylene/g of solid catalystcomponent. Said polypropylene has a fraction insoluble in xylene at 25°C. (X.I.)=96.9%, a meltindex R!L=5.2 g/10 min and an intrinsic viscosity[η]=1.7 dL/g.

EXAMPLES 2-10 AND COMPARATIVE EXAMPLES 11-13

The procedure of Example 1 is used, except that the 1,3-Diethers offormula (I) reported in table 1 are used as internal electron-donorcompounds. Also, in said table, are shown the composition of thecatalyst components and the polymerization results. TABLE 1 Mg Ti EtherActivity XI example 1,3-diether wt % wt % wt % Kg/gcat wt %  22-n-butyl-2-(3-pentyl)-1,3- 15.2 4.8 22 108 96.9 dimethoxypropane  32-n-pentyl-2-sec-butyl-1,3- 14.9 4.8 16.5 103 96.9 dimethoxypropane  42-n-propyl-2-cyclohexyl-1,3- 17.6 4.6 17.8 120 97.1 dimethoxypropane  52-n-butyl-2-cyclohexyl-1,3- 16.7 6.6 19.9 108 96.3 dimethoxypropane  62-n-pentyl-2-cyclohexyl-1,3- 14.7 4.8 19.2 95 96.8 dimethoxypropane  72-n-hexyl-2-cyclopentyl-1,3- 18.1 3.9 18.5 104 97.6 dimethoxypropane  82-i-butyl-2-cycloheptyl-1,3- 15.1 4.8 19.6 95 96.3 dimethoxypropane  92-i-pentyl-2-cyclopentyl-1,3- 16.7 4.7 17.9 101 97.2 dimethoxypropane 102-i-butyl-2-(1- 16.3 4.2 15.2 105 96.7 trifluoromethyl)ethyl-1,3-dimethoxypropane Comp. 11 2-i-pentyl-2-i-propyl-1,3- 19.4 3.6 16.8 8097.7 dimethoxypropane Comp. 12 2-i-butyl-2-i-propyl-1,3- 18.0 4.2 17.370 96.0 dimethoxypropane Comp. 13 2-i-propyl-2-cyclopentyl-1,3- 19.6 4.516.5 81 96.4 dimethoxypropane

1-17. (canceled)
 18. A 1,3-diether of formula (I)

in which R is a C₁-C₁₀ alkyl group, R₁ is a linear or branched primaryalkyl radical, optionally containing a heteroatom, and R₂ is acycloalkyl or (R₃)₂—CH— group, optionally containing a heteroatom, wherethe R₃ groups, same or different from each other, are C1-C10 linearalkyl radicals, provided that said R₃ groups are not contemporarily CH₃.19. The 1,3-diether according to claim 18 in which R₂ groups areselected from C₃-C₅ secondary alkyl radicals or C₅-C₇ cycloalkyl,optionally containing a heteroatom.
 20. The 1,3-diether according toclaim 18 in which the heteroatom is a halogen atom.
 21. The 1,3-dietheraccording to claim 18 in which R is a methyl group.
 22. The 1,3-dietheraccording to claim 18, wherein R₁ is n-butyl, n-pentyl, n-hexyl,n-heptyl, i-butyl or i-pentyl.
 23. The 1,3-diether according to claim 18in which R₂ is (1-trifluoromethyl)ethyl, sec-butyl, 3-pentyl,cyclopentyl, cyclohexyl or cycloheptyl.